1. Field of the Invention
The invention relates to a measurement arrangement having electrically heated resistors that are arranged in gas paths.
2. Description of the Related Art
DE 10 2009 014 618 A1 discloses a heat-conducting detector for use in gas analysis, where two electrically heatable heating filaments that comprise gold or platinum are mounted one behind the other in the middle of a channel such that gas can flow over the heating filaments, and for this purpose are each held at their ends on an electrically-conducting carrier that transverses the channel. In practice, the four heating filaments of two heat-conducting detectors of this type are connected to a Wheatstone bridge, where a measuring gas flows over one heat-conducting detector and a comparison gas or zero gas flows over the other heat-conducting detector and the heating filaments, over which in each case the same gas flows, lie diagonally opposite each other in the Wheatstone bridge.
WO 2009/153099 A1 discloses a microflow sensor, likewise for use in gas analysis, where two electrically heatable grid structures are arranged one behind the other in a gas path. The grid structures can comprise silicon or metal and are connected together with supplementary resistors to a Wheatstone bridge. The supplementary resistors can comprise the grid structures of a further microflow sensor.
In the case of a measurement arrangement disclosed in DE 102 30 198 A1, two electrically heated resistors together with two supplementary resistors are connected in a Wheatstone bridge, where the electrically heated resistors lie diagonally opposite each other and one of the supplementary resistors can be modified for the purpose of balancing the bridge. In order to eliminate offset voltages and slow signal drifts caused by temperature changes, the measuring voltage that is ascertained at the bridge is band-pass filtered.
EP 0 348 245 A2 illustrates a measurement arrangement having an externally-heated temperature-measuring resistor and three supplementary resistors in a Wheatstone bridge, where one of the supplementary resistors can be modified to balance the bridge.
DE 14 73 303 A discloses a measurement arrangement having two heating resistors for heating a measuring substance and a reference substance. The amount of heat absorbed by the respective substance is ascertained using two temperature-measuring resistors in a Wheatstone bridge. The energy supply for the heating resistors is controlled via the measuring voltage ascertained at the bridge.
For accurate measurements, it is important that the ratio of the resistance values of the resistors is identical in each of the two bridge halves. Otherwise, the bridge is out of balance. However, it is difficult to handle micromechanical measurement arrangements such as the known heat-conducting detector or microflow sensor during manufacture. The number of suitable resistors available for selection is limited by manufacturing tolerances and, owing to the sizes of the production batches, by a comparatively small number of available options.
It has been shown to be particularly problematic that the four resistors of the bridge can more or less greatly change over time. In addition, the changes, at least at the beginning, also occur in random directions. In other words, the values of some resistors reduce during the first operating days or weeks, others on the other hand increase. After a comparatively long running-in time, the values of all resistors then start to increase extremely slowly but in a monotone manner, until the resistors are destroyed at the end of their serviceable life. The direction of the changes that occur at the beginning cannot be forecast and therefore in the individual case can lead to the bridge being greatly out of balance for an extremely long period of time even if the bridge is comprised of selected high cost elements.